The basic model considers two independent stochastic processes: an ITO diffusion for the wood stock, Equation (1) and a geometric Brownian diffusion, Equation (2). The functional objective for the single rotation is given by Equation (3) and for the multiple rotations by Equation (4) see Johnson (2006) :

Model notation:

Deterministic state variable Economic Parameter

t = Wood stock age C = Stand regeneration cost

Stochastic sate variables P₀ = Price at time 0

V_t = Wood stock c = C/P₀

P_t = Wood stumpage price at time t R, Q = Probabilistic metrics

Diffusion Parameters F, Z = Functional objective

W_v = Wiener wood stock i = Risky rate of return

μ(V) = Wood stock diffusion drift rate parameter r_T =, Capitalized rate of return

σ(V) = Wood stock volatility parameter Optimal Parameters

W_p = Wiener price T = Optimal cut time

α = Wood price diffusion drift rate = Expected optimal cut volume

β = Wood price volatility

The stochastic model (1) (2) and (3) is difficult to solve due to both diffusions. But it is equivalent to the following one dimensional diffusion Optimal Stopping problem, (5) and (6) see Navarrete (2011) .

The solution of this problem is given by the (HJB) Equation (7), see Navarrete (2011) .

Assuming the existence of a frontier V^* that divides the zone into a continuation zone (no-cutting) and a stopping zone (immediate-cutting), the solution to the equation HJB is given by (8) for the continuation zone (8) and stopping zone (9). A solution of (8) is given by (10) (see Johnson, 2006 ).

where (resp.,) is strictly increasing (resp., decreasing), function since the payoff function are bounded and small, V is positive and should remain bounded and positive a V→0, necessarily then B→0. The solution also fulfils the smooth-pasting condition at the free boundary point V^*, equations (11) and eliminating the constant A gives Equation (12).

(12).

The Faustmann optimal rotation model (1) (2) and (4) its equivalent to the following Optimal Stopping one dimensional diffusion problem (13) and (14), see Lemma 1.

Lemma 1. Reformulation Lemma:

The Faustmann optimal rotation problem is equivalent to the following Optimal Stopping one dimensional diffusion problem (13) and (14).

Proof:

Given the independence of both variables the expectation can be calculated as the product of two independent expectation “” that are independent of the deterministic parameters and and the constant C, so they can be taken outside its domain. Introducing the integration of the price Brown diffusion

with and C = P₀c in the functional objective (4) results in the objective (15) and reduce the Faustmann problem to the following one dimensional diffusion problem (15) and (16).

Dividing the objective functional (15) by the constant P₀ and applying the Thijssen version of the Girsanov theorem to Equations (15) and (16), for the Martingale M_t and the Radom-Nykodym derivative (see Thijssen, 2010, Appendix A) permits to reformulate the problem (13) (14) by its equivalent Q metric optimal stopping problem (16) and (17).

The formulation of the HJB equation for this problem is given by Equation (17) for the capitalized risky rate of return see Navarrete & Bustos (2013) .

In this case the differential equation for the continuation region is given by the non homogenous differential Equation (18), and by Equation (19) for the stopping zone.

The solution to the ordinary differential equation, (18) under the initial condition for a given risky rate of return r_T is given in Equation (20), with, the positive increasing solution of the homogenous part and the particular solution of Equation (18).

In this case the smooth pasting condition for each parameter r_T are given by Equation (21):

So must fulfill smooth-pasting condition (22) for each parameter r_T.

The basic requirement of a pine stand growing diffusion is its sigmoid pattern (Garcia, 2005) . The logistic diffusion, Equation (23) is a special case of the sigmoid model given by and, where µ and γ are the drift and saturation parameters and σ is the volatility parameter.

The integration of the value of V is given by Equation (24) (Kloeden & Platen, 1992: p. 125) and its expected value is given by Equation (25)

Other important sigmoid diffusion is the Gompertz geometrical diffusion, which is given by Equation (26), which is integrated to the expression (27), and its expected values given by expression (28) (see Gutierrez, 2009).

The Geometric Brown Price diffusion is given by Equation (29), and integrates in Equation (30).

The experimental data belonged to 122 harvest pine stands of a Chilean Forest Company in the Araucania Chile region, between years 1999 and 2005 (see Appendix A), and came from different sample plots, with site indexes between 30 and 35 meters, representing sites with high forest aptitude and a tree average initial volume of 32 m³/ha at the first 4 years after initial seed cultivation period. The additional 20 point for years 11 and 12 were taken from Alvarez et al. (2012). The business harvest cut data for a Logistic diffusion for the 95% confidence range is given by data point plotted in Figure 1.

The logistic diffusion parameter cannot be adjusted by maximum verisimilitude, see (Beskos et al., 2006) , so it was fitted using a logistical nonlinear regression and a Monte Carlo/Bootstrap simulation sampling method, implemented by Meyer et al. (Loglet Lab.1 software, 1999) . Choosing V₀ = 1/2γ = half saturation volume, and T₀ = T_m, time to achieve that volume, Equation (25) is transformed into the more conventional expression (31).

With; 1/g = saturation volume, m = growth rate parameter, V_s = Saturation volume and t_m = time to achieve the midpoint of the saturation volume. The standard deviation Sd(∞), at the saturation zone is constant, and given by Equation (32).

Since the saturation volume V_s is also constant, sigma can easily be estimated by Equation (33) and the summary of the parameter fitting is shown in Table 1.

The Gompertz model can be fitted by common statistic features, such as maximum verisimilitude (see Gutierrez et al., 2008 ), but the lack of equal time distribution of data make it difficult. So a quadratic fitting method was development using the SSPS software. By taking natural logarithm and arranging it we get Equation (34).

with (35)

Given a value for k, a quadratic fitting for and was done estimating the value of A, B and C until a common value for θ was obtained from A and B, determining the estimation for θ, k and σ. The deterministic parameter only requires a linear fitting with. Both fittings were done for the initial value V₀ = 32 (m³/ha) and the results are summarized in Table 2.

The stumpage stand price Brownian diffusion parameters are estimated from saw logs and pulp logs exportation prices (see Appendix B). The summary of Brown diffusion parameters for the stumpage price and the regeneration cost of radiata Pine Stands are given in Table 3, see Navarrete & Bustos (2013) .

The deterministic optimal solution given by, for the simple rotation model and by equation (36) for the multiple rotation model, see Navarrete (2013).

Source: Appendix B.

The solution of the Vicksell and Faustmann model under Brown price and Logistic wood stock diffusion requires the solution of the differential Equations (8) and (19) for and. The positive increasing function ψ(V) for the Vicksell model, is the solution of the homogenous component (37) of the differential Equation (8) with r_T = i in the Vicksell case.

The solution of Equation (37) is given by the Kummer’s confluent hyper geometric function, expression (38) with the positive root θ by Equation (39), ψ(V) also must fulfill the smooth pasting condition (12), which was programmed in Mapple 15, see Navarrete (2013).

The Faustmann stochastic optimum requires the solution of the differential Equation (16), so the positive function ψ(V) is the solution of its homogenous Equation (37) for the parameter r_T and also fulfill the smooth pasting condition, (22). For a given value of the capitalized interest Equation (37) can be solved using the same methodology of the Vicksell solution, generating a family of optimal cuts for the different parameters r_T. The optimum solution is obtained intersecting the optimal parametric solution for different T with the corresponding expected value logistical modify diffusion, (14) for, see Figure 2. The summary of all optimal cuts results for the aggregate 30/35 site index series of the multiple and simple rotation harvest model under Brown price and Logistic wood stock diffusions is given in Table 4.

The simple optimal cut is a better explanation of the company cut policy, since it only underestimates it in a 2.5%. As expected the Faustmann optimal cut is lower and underestimate the company policy in 11.5%. The deterministic optimal cuts are even worse and underestimate the company policy in 23.4% in the simple model and in 25.6% in the multiple cases. Finally the Logistic wood stock underestimates the saturation Volume in 4.2%.

The deterministic optimal solution, is given by for the simple rotation model and for Equation (41) for the multiple rotation case the optimal is obtained intercepting condition (40) with Equation (27). See Navarrete (2013).

The solution of the Vicksell and Faustmann model under Brown price and Gompertz wood stock diffusion requires the solution of the differential Equations, (8) and (16) for and. The stochastic increasing function ψ(V), for these cases is given by the solution of the homogenous Equation (41) with r_T = i for the simple rotation case and the capitalized rate of interest r_T for the Faustmann case.

Replacing the parameters given in Equations (42) in (41), gives the differential Equation (43) of the Exponential, Ornstein-Uhlembeck diffusion whose positive increasing solution ψ(V) is given by Equation (44) (see Johnson, 2005), with:, b = 0.5 and

The optimal solutions also requires that ψ(V) fulfills the smooth pasting condition (38), which is programmed in Mapple 15 generating one optimal cut solution for the Vicksell model and a family of optimal cut condition parametrized by T for the Faustmann case. In this late case the optimal solution is obtain intercepting the family with the expected volume of the Gompertz modify diffusion (20), see Figure 3.

The deterministic and stochastic optimum were programed in Maple 15, using in this case the KummerU function of the program, the results are summarized in Table 5. These results show that the simple stochastic rotation model under Brown Price and Gompertz wood stock diffusion is a better explanation of the company cut policy, since it only overestimates it in 1.2%. As expected, the Faustmann model has lower optimal cut and underestimate it in 9.2% and the deterministic optimal cut is even lower, and underestimate the company policy in 27.9% in the simple case and in 33.1% in the multiple cases. Finally, the Gompertz wood stock overestimated the Saturation volume in 8.6%.